To Extract a Boundary Segment

Select the Extract Boundary tool.
Do one of the following:

Set Output As to LineStrings.

or
Set Output As to Points.
Set Extract to Boundary Segment.
Select the mesh element.
The mesh element highlights.
Click to accept the boundary.
The boundary highlights.
Select the first point on the boundary.
Select the second point on the boundary.
Do one of the following:

Click to extract the boundary segment.

or
Right-click to invert the boundary segment.